hey!! You can use 3.7v to 5v converter. If you have many phone batteries you can parallel connect them and add a tp4056 lithium battery charger for charging of the batteries . that idea would make a powerful rechargeable power bank (depends of the capacity the battery carries)

well it does not affect the batteries because you add it parallely I made one that was to you but more batteries, i have added 6 cells and hack my power bank to draw more ampere without affecting the circuitry it was good love the capacity it was almost 10k mAh